Most portable electronic devices provide the capability to operate from two or more input power sources. In addition to primary (non-rechargeable) batteries or secondary (rechargeable) batteries, a portable electronic device may use an auxiliary power source such as a wall adapter, supercapacitor, backup battery or Universal Serial Bus (USB) port.
Power from input power sources are supplied to one or more DC-DC converters provided in the portable electronic device to regulate the input voltage to precise bias levels required for microprocessors, miniature hard disk drives, digital circuitry, input/output rails, radio-frequency and audio circuitry, and other circuits. These DC-DC converters require the ability to select the most appropriate input power source in order to maximize their efficiency, prolong battery life and/or minimize heat generation.
In most cases, when a wall adapter or other non-battery power source is plugged into a portable device powered by a rechargeable battery, it supplies power in lieu of the battery to allow the battery to recharge in the shortest possible time. In the case of a portable device powered by a non-rechargeable battery, the auxiliary power source may provide power to retain volatile memory while the non-rechargeable battery is being replaced. When the auxiliary power source is removed, the portable device is powered by the battery.
Hence, a portable electronic device must rely on some input power selecting means that would make the switchover between the battery and the auxiliary power supply seamless. The switchover circuitry needs to insure that the input power sources are never directly connected to each other and that they block reverse current when an input power source is not selected. Also, it would be desirable to provide the switchover circuitry implementing an ideal diode function to avoid reducing system efficiency by placing additional resistances or diode forward voltage drops on the main power path.
Accordingly, there is a need for a circuit for selecting between input power sources that would maximize power efficiency and minimize heat generation to provide longer battery life in portable electronic devices. In addition, it would be desirable to create this circuit using the minimum number of components to improve manufacturability and cost.